1. Field of the Invention
The present invention relates to a method for performing a handover in wireless communication, a portable electronic device, and a wireless communication handover system.
2. Description of the Related Art
When a signal received from an access point (a base station), to which a wireless Local Area Network (LAN) device associates, becomes weak, the wireless LAN device becomes unable to maintain the communication. To assure that the wireless LAN link will be maintained, the wireless device switches to newly associate with another base station by performing a process by which the switching event is to take place, and which is called handover (or handoff).
A typical wireless LAN device has a single receiver unit; naturally, the wireless LAN device cannot search access points therearound while keeping the communication link via the currently serving access point. Therefore, the conventional wireless LAN devices release (dissociate) the association when the strength of the signal received from the base station with which the device currently associates is under a certain level, then scans all the available wireless LAN channels, searching for an access point from which the device receives a strong signal, and associates with the detected access point.
The wireless LAN device once releases the current association, and only then it scans all the channels to find a new access point. In this scenario, the wireless LAN device needs a substantial time to complete the handover. Moreover, the wireless LAN device will again be associated with the originally associated access point when it fails to find a new access point, rendering the series of processes end up in the waste.
On the other hand, a wireless LAN device that affords cutting off time to conduct a handover is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2005-175932. The disclosed wireless LAN device, on detecting that the level of the signal which it receives from the currently associated base station is smaller than a predetermined value, temporarily interrupts the communication with the currently associated access point, scans the channels and newly associates with a destination after detecting an appropriate channel.
This wireless device stops the communication with the currently associated access point and scans all the channels. Taking a substantial time to scan all the channels, therefore, this wireless LAN device sees large data loss in communicating with the currently associated access point.
A wireless LAN device, which requires shorter time for handover operations, is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2006-270614. This wireless LAN device, keeping association with one access point, measures the strength of the signals received from other channels, attempts to perform a handover only to a channel whose signal strength is greater than a threshold value, and stops the subsequent handover operations once it has successfully performed a handover. This wireless LAN device also sees large data loss in communicating with the currently associated access point because this wireless device, similarly to the above-mentioned devices, scans all the channels while temporarily stopping the communication with the associated channel.
Another wireless LAN device is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2006-262178, in which the wireless terminal device measures the quality of communication being performed with the adjacent systems for which an order of precedence is determined in advance, and performs handover based on the order of precedence and the measured quality of communication. In advance of the handover, this wireless device measures the quality of the communication being performed by each of the adjacent systems. Therefore, this wireless LAN device also faces the similar problem of data loss in communicating with the currently serving system.
Various mobile terminal devices, not limited to wireless LAN devices, have problems of this kind, which are commonly seen in those mobile terminal devices performing wireless communication via an access point (base station).